@Shoichi, the question is when to do this in Spanish, rather than how. Since I'm not fluent in Spanish and speak it about as well as the person who posted the question, I suggested he put the question on the Spanish forum so he will be more likely to get a good answer. In English, I've never seen a melisma between two words in what appears to be an arbitrary location. Perhaps it's because of the diphthong (two vowels with one sound), but I wouldn't guarantee it. I know a Spanish speaker who says "tengo una" would turn the o and u into a diphthong rather than pronouncing the o then the u.